Various types of optical spectrometers are in use for such purposes as atomic emission spectroscopy, atomic absorption spectroscopy and astronomy. A complete system generally consists of a source of radiation, a spectrometer for separating and detecting individual spectral components, and a data station for processing the information from the spectrometer. The radiation source, for example, may be a system for injecting a test sample into an inductively coupled plasma where the atomic species in the sample are excited to radiate characteristic atomic emission. As another example, a sample is evaporated in a graphite furnace where the gaseous sample absorbs certain frequencies of the incident radiation to provide atomic absorption lines. Similarly, astronomical sources provide atomic emission and absorption lines.
Spectrometers are based on dispersion of radiation by diffraction gratings, prisms and combinations of the two. Generally electronic detection devices are taking over from photographic film for accurate and timely measurements of the emission or absorption lines. A major objective in the development of spectrometers is improvement in detection devices in order to gain sensitivity, dynamic range, signal/noise ratio and speed in the quantitative measurement of atomic species in a test sample or other source.
Spectrometers are frequently designed around available detector technology. There are basically two classes of spectrometers. One involves sequential measurement utilizing a monochromater in which a grating or prism is rotated. The angle is adjusted to correspond to the different emission (or absorption) lines of the elements. A single detector is used, and a measurement process involves relatively rapid rotation of the grating with measurements at a fixed location corresponding to grating angles appropriate to the atomic emission lines.
Spectrometers of the other class are direct readers, in which a full spectrum is displayed across some form of detection system which is capable of detecting the individually focused spectral lines. According to current technology, the best sensitivity is attained by providing a slit for each of the several emission lines being measured, with a photomultiplier tube placed on the opposite side of each slit so as to detect each line. In practice the number of slits with tubes is limited by the size and cost of photomultiplier tubes, so a different slit structure must be used for different types of samples, and there must be some preliminary knowledge of sample composition for choice of slit location.
Since background radiation is generally present, there also must be some method for measuring background in order to correct the emission data. Background measurement presently is effected before and/or after the emission detection. In a sequential system background may be measured for monochromater grating angles proximate those for the atomic emission lines. For direct readers the background is generally measured by shifting the position of the entrance slit and making consecutive measurements.
One of the most sensitive types of spectrometer in current use is an echelle spectrometer which provides a display of spectral lines in two dimensions. This spectrometer and its principles are described in "The Production of Diffraction Grating: II. The Design of Echelle Gratings and Spectrographs" by G. R. Harrison, J. Opt. Soc. Am. 39, 522 (1949). Details of such a system are given in a paper "Echelle Spectroscopy with a Charge-Coupled Device (CCD)" by D. G. York, E. B. Jenkins, P. Zucchino, J. L. Lowrance, D. Long and A. Songaila, SPIE Vol. 290 Solid State Imagers for Astronomy, 202 (1981). Briefly, light passing through an entrance slit is collimated and directed to an echelle grating which has a low density of shaped grooves to produce high order diffraction patterns. The diffracted beam is directed to a second, crossed grating with a higher density of grooves, or a prism, which separates the orders into a two-dimensional pattern. This pattern is focused onto a two-dimensional detecting surface which is configured to detect the individual spectral lines.
There are two types of practical electronic photodetectors. Photomultiplier tubes are quite sensitive but are relatively large and, therefore, are physically incapable of being assembled to detect a number of adjacent lines. Also, a large number of photomultiplier tubes becomes quite expensive.
The other type of photodetector is solid state, based on the principles of charge generation upon the incidence of radiation on a surface such as silicon. To provide resolution of spectral lines (or, more broadly, image resolution) such a surface on a semi-conductor chip is divided into pixel areas. The accumulation and handling of signals from the pixels is effected through the transfer of charges in the chip from the pixels. The technology is detailed, for example, in the book "Charge Transfer Devices" by C. H. Sequin and M. F. Tompsett, Academic Press (1975). Of particular interest are pages 11-14, 19-42 and 142-146 wherein there are described charge-coupled devices (CCD) and their use in image sensing.
A related approach for such detectors is charge injection device (CID) technology. This is described in the article "Review of Charge Injection Device (CID) Technology" by A. B. Grafinger and G. J. Michon, SPIE Vol 244 Mosaic Focal Plane Methodologies, 26 (1980).
Image sensing CCD's and CID's were developed primarily for video cameras involving full area coverage of the image plane. These have been incorporated into spectrometers and other optical systems for astronomy purposes and have been quite useful. The latter use in echelle spectroscopy is presented in the aforementioned paper by York et al., where a 512.times.320 pixel CCD is described. A problem presented in the paper is high readout noise for such a device. The video type of area detectors have shown limitations in sensitivity associated with the noise that results from the high multiplicity of signals from the full array of pixels on the surface as well as from the high density of solid state channeling of charges and signals in the device.
Further problems with conventional CCD's and CID's include poor sensitivity to ultraviolet radiation due to absorption by the conductor channels, for example polysilicon, which perform the charge transfer; high noise due to the relatively large values of capacitance associated with the conductor channels from all pixels; high data rates for readout which causes high noise; long readout times for large numbers of pixels; limited dynamic range in the ability to handle different light intensities due to saturation of charge at one pixel and consequent spreading of charge into adjacent pixels; and difficulty in obtaining random access of a large number of pixels.
Therefore, a primary object of the present invention is to provide a novel solid state array detector useful in an optical spectrometer of the type that produces a two dimensional display of spectral lines.
A further object is to provide a novel solid state detector of two dimensional spectra that has reduced noise, improved sensitivity, improved dynamic range, reduced readout data rates and random access of pixels for readout.
Another object is to provide a highly sensitive detector of a two dimensional spectral display at reasonable cost.